Embryonic Development
VARIATIONS IN EMBRYONIC GERM LAYERS AND BODY CAVITY
Formation of the Digestive Cavity

At the Blastula stage, the embryo forms a fluid filled ball of cells

During Gastrulation, the blastula caves in on one end to form a
Gastrula. The cavity formed by gastrulation will form a digestive
cavity or digestive tract.

If gastrulation is incomplete, the gastric cavity will have only one
opening. This arrangement is characteristic of Cnideria and
Platyhelminthes

If gastrulation is complete, the digestive tract will have two openings.
This arrangement has 2 variations, protostomes and deuterostomes
Protostomes vs. Deuterostomes
In Protostomes, the blastopore forms the mouth. In Deuterostomes,
the blastopore forms the anus and the mouth is the second opening
Embryonic Germ Layers

Organisms that undergo gastrulation fit
into 2 categories: diploblasts and
triploblasts

Diploblasts only have 2 embryonic
germ layers

Gastrulation forms an inner layer of
cells (endoderm) and an outer layer
(ectoderm)

This diagram shows the endoderm in
blue and the ectoderm in black
Embryonic Germ Layers

Triploblasts develop a third germ layer
in between the endoderm and the
ectoderm

The middle layer is called mesoderm

In the diagram, mesoderm is
represented in yellow

Variations in the presence, absence
and arrangement of mesodermal tissue
is one of the most important
distinguishing features of animal phyla
Phylogeny based on Mesodermal
Arrangement

Phylum porifera lack a
true digestive cavity

Cnideria are
diploblastic. They have
a digestive cavity but
lack mesodermal tissue

The triploblastic phyla
vary in regard to the
arrangement of
mesoderm and body
cavity
Development of Mesodermal Tissue

Mesoderm is formed by a migration of
endodermal cells into the blastocoel.

Mesoderm forms by one of 2 possible
mechanism

The diagram at the right shows
mesoderm development in acoelomates
and protostomes

The formation of mesoderm initiates near
the blastopore. Dividing mesodermal
cells migrate into the blastocoel
Acoelomate phyla

Acoelomates do not form a body
cavity

Flatworms (Platyhelminthes) are
acoelomate

Mesodermal cells fill the entire
blastocoel

Internal organs form, but are not
separate and distinct
Pseudocoelomate Phyla

Pseudocoelomates have a body cavity,
but mesoderm is associated with the outer
body wall (ectoderm).

Nematodes and rotifers are
pseudocoelomate

All pseudocoelomates have a true
digestive tract (2 openings) and are
protostomes (blastopore = mouth)

The digestive tract lacks mesodermal
tissue, thus is not muscular
Cross Section of a Pseudocoelomate

The diagram shows a cross section
through a nematode

Notice the space between organs
(body cavity), but note that there is
only mesoderm on the outer
boundary, associated with the body
wall. The Intestine is a simple layer of
epithelium (no muscle)

The organs are separate and distinct,
but the body cavity is “false” – not fully
lined with mesoderm
Coelomate Protostomes

A true body cavity is fully lined with
mesodermal tissue.

In coelomate protostomes, the body cavity is
“schizocoelous” (note the descriptive root
word “schizo”)

The solid mass of mesoderm filling the
blastocoel splits due to the programmed cell
death (“apoptosis”) of some centrally located
cells

The diagram shows the coelom in pink
Coelomate Deuterostomes

All deuterostomes have a true body cavity

In deuterostomes, the blastopore forms the
anus (so the “second opening” forms the
mouth”)

In deuterostomes, the mesoderm forms
from outpouching of the gut

Since it is “caving in” it is “enterocoelous”

Because the coelom is formed from
pouches, it is completely surrounded by
mesodermal tissue
Schizocoelous vs Enterocoelous